Unidirectionality of a unidirectional dynamic microphone is achieved by a combination of an omnidirectional component and a bidirectional component. The omnidirectional component depends on an acoustic resistance, while the bidirectional component depends on a mass control. Implementation of the unidirectionality requires a flat frequency response of the resistance component. A unidirectional dynamic microphone thus includes an acoustic resistor provided immediately behind the diaphragm.
FIG. 8 is a longitudinal cross-sectional view illustrating a typical conventional unidirectional dynamic microphone unit 200. In FIG. 8, a body frame 1 functions as a base of the microphone unit. The body frame 1 functions as a part of a magnetic circuit described below: the body frame 1 serves as an outer yoke. The body frame 1 is a substantially cylindrical member of a magnetic material, and is provided with a center hole. The diameter of the substantially lower half of the center hole is smaller than that of the upper half as shown in FIG. 8. The body frame 1 also has a step on the inner circumference wall in the middle of the vertical direction.
The step in the center hole of the body frame 1 is fixed to a disc yoke 2. The yoke 2 has a disc magnet 3 fixed thereupon. The magnet 3 has a disc pole piece 4 fixed thereupon. The yoke 2, the magnet 3, and the pole piece 4 respectively have center holes 21, 31, and 41 in the same diameter. The body frame 1, the yoke 2, the magnet 3, and the pole piece 4 are connected to each other by bonding. The body frame 1 serves as the outer yoke as described above, while the yoke 2 serves as an inner yoke. The outer circumference wall of the yoke 2 is in close contact with the inner circumference wall of the body frame 1. A circular gap in a plan view is defined between the outer circumference wall of the pole piece 4 and the inner circumference wall of the body frame 1. A magnetic flux originated from the magnet 3 passes through a magnetic circuit consisting of the yoke 2, the body frame 1 serving as the outer yoke, the gap, and the pole piece 4, and returning to the magnet 3, which allows the gap to function as a magnetic gap. The outer diameter of the magnet 3 is smaller than that of the pole piece 4, which defines an air chamber 9 surrounding the magnet 3, the air chamber 9 having a larger width compared to the magnetic gap and located beneath the magnetic gap.
The top end of the body frame 1 is surrounded by a cylindrical member 35 fixed thereto. The top end of the cylindrical member 35 includes an internal flange 36 on the inner circumference wall. The flange 36 is fixed to the outer circumference wall of the top end of the body frame 1 by, for example, bonding. The flange 36 has a plurality of vertical through holes 37. A cylindrical space between the inner circumference wall of the cylindrical member 35 and the outer circumference wall of the body frame 1 is in communication with a space above the cylindrical member 35 through the holes 37. The tops of the holes 37 are covered with an acoustic resistor 18.
The diaphragm 5 has an outer peripheral edge fixed to the top end of the cylindrical member 35. The diaphragm 5 consists of a center dome 51 and a sub dome 52 surrounding the center dome 51 that are produced by shaping a thin film of a synthetic resin or metal. The center dome 51 has a partially spherical shape. The sub dome 52 has a partially arc-shaped cross-section and surrounds the peripheral edge of the center dome 51. The sub dome 52 has an outer peripheral edge fixed to the outer peripheral edge of the cylindrical member 35. The diaphragm 5 including such a sub dome 52 is fixed to the cylindrical member 35, the outer peripheral edge of the sub dome 52 being the fixed point as described above. Upon receipt of sound waves, the diaphragm 5 can vibrate in the anteroposterior direction (in the vertical direction in FIG. 8) in response to sound pressure, the fixed point or the outer peripheral edge of the sub dome 52 functioning as a node.
The diaphragm 5 has a voice coil 6 fixed along a boundary between the center dome 51 and the sub dome 52. The voice coil 6 is a fine wire wound into a cylindrical shape and fixed to maintain a steady shape. The first end of the cylindrical voice coil 6 is fixed to the diaphragm 5. While the outer peripheral edge of the sub dome 52 of the diaphragm 5 is fixed in such a manner as described above, the voice coil 6 is located in the magnetic gap. The voice coil 6 is separated from the body frame 1 and the pole piece 4. The sub dome 52 of the diaphragm 5 covers the tops of the holes 37 of the cylindrical member 35 and the upper surface of the acoustic resistor 18.
Adjacent to the reverse of the diaphragm 5 (beneath the diaphragm 5 in FIG. 8), a protector 7 is fixed to the top of the pole piece 4. The protector 7 has a dome top portion. The protector 7 keeps a certain distance from the center dome 51 of the diaphragm 5. The protector 7 has a center hole 71 in communication with the center holes 41, 31, and 21 of the pole piece 4, the magnet 3, and the yoke 2, respectively.
Adjacent to the obverse of the diaphragm 5, an equalizer 8, which serves as a protector for the diaphragm 5, is located. The outer peripheral edge of the equalizer 8 is fixed to the outer peripheral edge of the top end of the cylindrical member 35. The equalizer 8 has a dome ceiling in the center. The equalizer 8 keeps a certain distance from the center dome 51 of the diaphragm 5. The equalizer 8 has a center hole 81 and a plurality of holes 82 around the center hole 81 through which sound waves propagate from the outside to the diaphragm 5.
An opening at the bottom end of the body frame 1 is covered by a cap 10, which defines a relatively large air chamber 11 at the bottom end of the body frame 1. The air chamber 11 holds an acoustic resistor 12 of a thickly-layered nonwoven fabric therein. The acoustic resistor 12 is disposed adjacent to the reverse of the diaphragm 5. Such a configuration provides a unidirectional dynamic microphone unit.
Upon receipt of sound waves, the diaphragm 5 vibrates in the anteroposterior direction in response to changes in sound pressure. The vibration of the diaphragm 5 induces anteroposterior vibration of the voice coil 6. During the vibration, the voice coil 6 moves in the magnetic flux across the magnetic gap. The voice coil 6 moving in the magnetic flux generates voice signals in response to changes in sound pressure. The dynamic microphone unit electro-acoustically converts the signals as described above. The voice signal is output, for example, from the both ends of the voice coil 6 fixed along the inner peripheral edge of the rear surface of the sub dome 52.
The microphone unit having the configuration described above resonates with acoustic masses and acoustic capacitances formed at various sites. The cause of resonance is described below. The magnet gap defined between the inner circumference wall of the body frame 1, which serves as the outer yoke, and the outer circumference wall of the pole piece 4 is as small as possible, provided that the voice coil 6 does not come into contact with the surrounding portions. Such a small magnetic gap is provided to increase the magnetic flux density in the magnetic gap in order to achieve high sensitivity of a microphone. The voice coil 6 thus substantially partitions a space adjacent to the reverse of the diaphragm 5 into an acoustic capacitance space Sc adjacent to the reverse of the center dome 51 and an acoustic capacitance space Ss adjacent to the reverse of the sub dome 52.
The voice coil 6 partitions each of the acoustic mass and the acoustic resistance of the magnetic gap into an inner section and an outer section. An acoustic mass mgi and an acoustic resistance rgi are generated in the inner section or a space between the inner circumference wall of the voice coil 6 and the outer circumference wall of the pole piece 4. An acoustic mass mgo and an acoustic resistance rgo are generated in the outer section or a space between the outer circumference wall of the voice coil 6 and the inner circumference wall of the body frame 1 serving as the outer yoke. The air chamber 9 between the inner circumference wall of the body frame 1 and the outer circumference wall of the magnet 3 holds an acoustic capacitance Sg. The acoustic capacitance space Sc is in communication with the acoustic capacitance space Ss through the acoustic mass mgi, the acoustic resistance rgi, the acoustic capacitance Sg, the acoustic mass mgo, and the acoustic resistance rgo.
The obverse of the diaphragm 5 undergoes a sound pressure P1. The acoustic resistor 12 disposed in the air chamber 11 of the body frame 1 has an acoustic resistance r1. The air chamber 11 holds an acoustic capacitance S1. The hole 37, which extends vertically through the flange 36 of the cylindrical member 35 and is in contact with the acoustic capacitance space Ss adjacent to the reverse of the sub dome 52, has an acoustic mass m1. The sound waves propagating through the hole 37 has a sound pressure P2. The acoustic resistor 18 between the hole 37 and the acoustic capacitance space Ss holds an acoustic resistance r2. The air chamber adjacent to the obverse of the diaphragm 5 holds an acoustic mass mo and an acoustic capacitance So.
FIG. 10 shows an equivalent circuit of the microphone unit illustrated in FIG. 8, the microphone unit having the acoustic mass, the acoustic capacitance, and the acoustic resistance as described above. In FIG. 10, the sound pressure P1, the acoustic mass mo, the acoustic capacitance So, the acoustic mass mgi, the acoustic resistance rgi, the acoustic resistance rgo, the acoustic mass mgo, the acoustic mass m1, the acoustic resistance r2, and the sound pressure P2 are in series connection. A node between the acoustic capacitance So and the acoustic mass mgi and a first node between the sound pressure P1 and the sound pressure P2 are connected to the acoustic capacitance Sc. These nodes are also connected to the acoustic resistance r1 and the acoustic capacitance S1 that are in series connection. A node between the acoustic resistance rgi and the acoustic resistance rgo and a second node between the sound pressure P1 and the sound pressure P2 are connected to the acoustic capacitance Sg. A node between the acoustic mass mgo and the acoustic mass m1 and a third node between the sound pressure P1 and the sound pressure P2 are connected to the acoustic capacitance Ss.
FIG. 10 demonstrates that the acoustic mass mgi of the inner section of the magnetic gap partitioned by the voice coil 6 and the acoustic capacitance Sg of the air chamber 9 define a resonance circuit. The acoustic mass mgo of the outer section of the magnetic gap and the acoustic capacitance Ss of the space adjacent to the reverse of the sub dome 52 also define a resonance circuit. The volume of the air chamber 9 is smaller than the air chamber 11 at the half bottom of the body frame 1. The acoustic capacitance Sg of the air chamber 9 readily causes a resonance in cooperation with the acoustic mass mgi.
FIG. 11 is a spectrum representing frequency characteristics of observed sound waves from the microphone unit forming such a resonance, i.e., frequency characteristics of the sound waves from an angle of 0° with respect to the microphone unit or from the direct front of the microphone unit, the sound waves from an angle of 90° with respect to the microphone unit or from just beside of the microphone unit, and the sound waves from an angle of 180° with respect to the microphone unit or from immediately behind the microphone unit. FIG. 11 evidently shows that the sound waves that have respective angles with respect to the microphone unit achieve resonance peak at a specific frequency, which leads to unsatisfactory frequency characteristics. A unidirectional microphone desirably outputs signals at a flat level over a wide frequency range from low frequency to high frequency. The conventional unidirectional microphone outputs signals at variable levels depending on the frequency as shown in FIG. 11. The microphone having such frequency characteristics may provide uneven directivity, and convert sound waves from a specific direction into electrical signals representing different sound tones.
A possible measure to prevent such a resonance with the acoustic mass mgi is a further reduction in volume of the air chamber 9 to minimize the acoustic capacitance Sg. The typical conventional dynamic microphone unit 300 illustrated in FIG. 9 includes such an extremely small air chamber. The microphone unit 300 has a spacer 15 surrounding the magnet 3. Like the typical conventional dynamic microphone unit illustrated in FIG. 8, the dynamic microphone unit 300 illustrated in FIG. 9 has the air chamber 9 between the outer circumference wall of the magnet 3 and the inner circumference wall of the body frame 1, but the most part of the air chamber 9 is filled with the spacer 15. The outer circumference wall of the half of the top end of the spacer 15 is cut away to form a step. The step faces the bottom end of the voice coil 6 and keeps a distance from the voice coil 6, and the distance functions as a clearance for the stroke of the voice coil 6. The step of the spacer 15 defines a space corresponding to the air chamber 9 having an extremely small volume. Such an air chamber 9 thus holds an extremely small acoustic capacitance, which prevents a resonance with the acoustic mass mgi.
Contrarily, an effective measure to improve the sensitivity of a microphone unit is an increase in an effective area of the diaphragm 5 or the main dome 51. As the area of the dome 51 increases, the diameter of the voice coil 6 increases, which expands the stroke of the diaphragm 5 or the voice coil 6. Accordingly, the air chamber 9 accommodating the voice coil 6 must have larger volume, which hinders the reduction of the acoustic capacitance Sg of the air chamber 9. The typical conventional microphone unit illustrated in FIG. 9 having an outer diameter of, for example, about 40 mm limits the reduction in the volume of the air chamber 9. Hence, the typical conventional microphone unit illustrated in FIG. 9 cannot effectively prevent the resonance caused by the acoustic mass mgi and the acoustic capacitance Sg.
A microphone unit described in Japanese Unexamined Patent Application Publication No. H11-275680 includes a frame yoke having an opening as a directivity circuit on a flange portion. Behind the opening, a small-volume air chamber is disposed via a directivity resistor, the air chamber having a front side, which opens to and faces the opening, and a back side having a small opening. Such a configuration defines a resonance circuit. The resonance circuit causes a resonance at a target frequency, which reduces the acoustic impedance of the directivity circuit, and increases sound pressure difference between 0° characteristics or front characteristics and 180° characteristics or rear characteristics in order to avoid howling.